Super Reverb Vibrato Delay

[Daver]

The zero, lead only resistance is .4 ohms. The meter is a Fluke. I'm not at home now so I don't have the model number. The board is brand new. I started from scratch.

[R.G.]

And now a word from the theoretical section.

The oscillator in the amp is a phase-shift oscillator. Phase shift oscillators oscillate at the frequency where the phase shifts around the gain loop (that's those caps and resistors feeding back to the grid of the oscillator tube) equal 180 degrees, and where the overall gain from the grid of the oscillator tube to its plate, and back through the lossy capacitor/resistor network is equal to or greater than one.

However, the startup time, how long oscillations take to build up, is determined by that loop gain. If the loop gain is ***exactly*** unity, it will sustain an oscillation, perhaps started by noise, but never build up to full size. If the loop gain is less than unity, it will ring like a bell, but never sustain an oscillation. If it's greater than one, the oscillation will build up at a rate determined by the "excess gain" over unity. With high excess gains, startup is almost instantaneous, but there will be distortion.

So if the oscillator takes too long to start up, either (1) the tube doesn't have enough gain, perhaps because of a faulty cathode cap, high drifted cathode resistor, etc., or (2) the feedback loop from plate to grid is attenuating the signal too much, causing too much of the signal to be lost before getting back to the grid.

That's why the values of resistance, footswitch cabling, contacts, and so on affect the start up time. There is another issue, and that is that the feedback network has a variable loss at different frequencies, so the LFO will oscillate better at some speed settings than others, generally at the higher ones. That might be a good thing to check.

[Daver]

Thanks R.G. I have tried numerous new stock and NOS tubes with no variation in how it acts. All the resistors and caps are new, measure in spec and then were all replaced again. How would the feedback loop be attenuating the signal too much? Are you thinking like drifted resistors and caps or something else? When you're talking about checking it at higher speeds, do you mean to start it with the speed turned up to start?
I tried it again today. When I hit the footswitch, it didn't start. After about 5 seconds I shut it off before it started. I then removed the footswitch plug from the amp and stuck in my jumper plug. It started right up. It keeps looking like the footswitch even though it seems to be grounding when I measure it. Thanks everyone.

[Daver]

Well I wired up a bare footswitch to my jumper plug and used that instead of the regular footswitch. The amp is sitting loose in the cabinet at this point. So I verify I have good continuity to ground when the switch is engaged, turn it off, switch on the amp and play for 30 seconds or so. Then I hit the switch and what did I get? Ticking. No vibrato just ticking for 20 seconds and then the vibrato started. Aaahhh.... Another new wrinkle. And, yes, I did rebuild the opto side of the vibrato circuit when I did the oscillator. That has to be a clue. Why would it tick with no vibrato? Thanks.

[52 Bill]

Why would it tick with no vibrato? Thanks.

Does the neon lamp flash when it does this?

[Daver]

I don't know. It's completely sealed. I'll have to put a cut in the shrink tubing to see it. If the bulb were not coming on, would it make the ticking noise? Thanks.

[Daver]

OK it works now. I'm not 100% sure why. I cut open the bulb/ldr shrink tube to see if the bulb was blinking when I heard just ticking. The vibrato worked. I moved the bulb a bit in relation to the ldr because it didn't seem to be aligned well. I then put some silicone glue on it to hold it in place and glued down the shrink tube I cut back. It has since continued to work flawlessly for days in its own cabinet using the footswitch. I can't believe I had two bad roaches from two different places. Further, I wouldn't expect that to cause a delay in the vibrato. I would think it would just not work or be weak. Go figure. I can let it sit for 24 hours and it will fire right up. Thanks for all the help.

Dave

[TimmyP1955]

Could be that the legs in the bug were touching, and by cutting it open you disturbed them enough to solve the issue. I recently had a (I forget) that had a nasty crackle at volume. When chopsticking, I found it to be the bug and opened it - the legs were apparently so close that they would touch when there was enough vibration - I separated them and no more problem.

[Daver]

Could be. It's just odd that two different ones did the same thing. It's been flawless for a week now.